1. Field of the Invention
The present invention is generally related to viscoelastic damping treatments and, more particularly, to a damping treatment which utilizes fibers dispersed throughout a viscoelastic medium to increase shear strain without the need for a constraining layer.
2. Description of the Prior Art
It is well known that constrained viscoelastic layer damping treatments provide an effective means of passive control for structural vibration. The fundamental principle behind viscoelastic damping treatments is the conversion of structural vibration energy into heat by inducing strain in the viscoelastic layer. Viscoelastic damping is exhibited strongly in many polymeric materials. Polymeric materials are made up of long molecular chains. The damping arises from the relaxation and recovery of the polymer network after it has been deformed. As shown in FIG. 1A, a typical damping treatment is comprised of a viscoelastic layer 3 which in turn has an elastic constraining layer 5 bonded to a top surface 7 to form a "sandwich-like" arrangement 9. The arrangement 9 is often manufactured as a tape, having an adhesive surface 11 which may be bonded to the surface of a structure 13.
The constraining layer 5 is much stiffer than the viscoelastic layer 3, so that when the structure 13 vibrates, the constraining layer 5 experiences relatively small deformations, thereby inducing shear strain in the viscoelastic layer. FIG. 1B illustrates that when the substrate material 13 experiences an axial deformation 15, the viscoelastic layer 3 experiences shear strain. Thus, the constraining layer 5 leads to a substantial net increase in viscoelastic layer strain energy as compared to the unconstrained case. Since the structural damping contributed by the structure 13 and the constraining layer 5 is very small, it is generally assumed that the energy lost is related to the shear strain of the viscoelastic layer 3 alone. This lost strain energy is converted to heat and dissipated to provide the desired mechanical damping for the structure 13.
Constrained layer damping treatments most commonly use spatially continuous constraining layers over the entire viscoelastic layer. FIG. 2A shows a continuous constraining layer 25 bonded to a viscoelastic layer 23 to form a tape 27 which, in turn, is bonded to a beam 33 subjected to a bending deformation. However, it is also well-known that in some cases it is the constraining layer length which influences the resultant damping more than the amount of damping material used, as discussed by Plunkett and Lee, "Length Optimization for Constrained Viscoelastic Layer Damping" J. Accoustic Soc'y of Am., Vol. 48, No. 1, at 150-61 (1970). For example, in FIG. 2A, if the constraining layer is very long, then the stress imposed by its endpoints 29 will induce axial strain in most of the constraining layer 25 that is nearly equal to that of the base structure 33. Accordingly, relatively little shear is induced in the viscoelastic layer 23 except near the constraining layer's endpoints 29. Because the greater part of the viscoelastic layer 23 is not undergoing shear strain, the structure 33 is not effectively damped. This condition may be remedied by segmenting the constraining layer 25, as shown in FIG. 2B, into sections 31 of prescribed length 34 such that optimal damping is achieved at a specified design frequency. However, length optimization is generally a tedious method which requires the application of a viscoelastic layer to a surface followed by the careful application of sections of constraining layer on top of the viscoelastic layer.
If multiple layers of polymers which exhibit viscoelastic properties are employed as illustrated in FIG. 3, the damping can be further increased. When the structure 13 vibrates, the layers of constraining material 5 create a large magnitude shearing force throughout the layers of viscoelastic material 53 which are sandwiched in-between the structure 13 and the constraining layers 5. Because there is an increase in the volume of viscoelastic material 53 which is subject to shearing deformation, the net strain energy in viscoelastic material 53 is increased and, therefore, improved damping in achieved. However, with increasing layers of viscoelastic material 53, both the weight and size of the treatment increases, and additional constraining layers 5 must be employed.